Mechanical Properties of Three-Dimensional Printed Provisional Resin Materials for Crown and Fixed Dental Prosthesis: A Systematic Review

The emergence of digital dentistry has led to the introduction of various three-dimensional (3D) printing materials in the market, specifically for provisional fixed restoration. This study aimed to undertake a systematic review of the published literature on the Mechanical Properties of 3D- Printed Provisional Resin Materials for crown and fixed dental prosthesis (FDP). The electronic database on PubMed/Medline was searched for relevant studies. The search retrieved articles that were published from January 2011 to March 2023. The established focus question was: “Do provisional 3D-printed materials have better mechanical properties than conventional or milled provisional materials?”. The systematically extracted data included the researcher’s name(s), publication year, evaluation method, number of samples, types of materials, and study outcome. A total of 19 studies were included in this systematic review. These studies examined different aspects of the mechanical properties of 3D-printed provisional materials. Flexural Strength and Microhardness were the frequently used mechanical testing. Furthermore, 3D-printed provisional restorations showed higher hardness, smoother surfaces, less wear volume loss, and higher wear resistance compared to either milled or conventional, or both. 3D-printed provisional resin materials appear to be a promising option for fabricating provisional crowns and FDPs.


Introduction
The emergence of computer-aided design and computer-aided manufacture of CAD/CAM in dentistry facilitates the production of definitive crowns and FDP in a timely manner [1]. Unless the definitive prosthesis is delivered in the same visit, patients must be provided with a provisional restoration from the initial tooth preparation until the definitive prosthesis is placed [2]. Provisional restorations are an essential step in fixed prosthodontic treatment [3]. They are designed to enhance esthetics, function, and assess the effectiveness of a specific treatment plan [4]. Multiple clinical scenarios require a provisional phase, such as full mouth rehabilitation, immediate loading for implants, and crown lengthening cases [5,6].
Basic requirements for provisional restorations can be divided into biological, biomechanical, and aesthetic requirements [3]. They should be biocompatible, non-irritant, have a pleasant odor and taste, and provide a highly polished surface [4]. They must be strong, durable, hard, wear-resistant, and able to withstand the functional forces of mastication without fracture or displacement [4,7]. There are many methods, techniques, and materials to fabricate the provisional materials [8]. According to the chemical composition of provisional materials, they can be classified into two main groups [9]. The first group is dental professionals interested in learning or using 3D-printed provisional materials as a helpful guide in daily clinical practice. Conducting this systematic review would provide a clear, unbiased comparison between all three fabrication methods (conventional, milled, and 3D-printed) for provisional fixed prostheses.
Therefore, this study aimed to undertake a systematic review of the published literature on the mechanical properties of 3D-printed provisional materials for crowns and FDP. We conducted this review to test the null hypothesis that there was no significant difference in mechanical properties between 3D-printed provisional prostheses materials and the currently available milled and conventional provisional fixed materials. The underlying patient or problem, intervention, comparison, and outcome (PICO) question was: "Within the available studies in the literature, do provisional 3D-printed fixed materials have mechanical properties comparable to conventional or milled provisional fixed materials?". The primary outcome was the mechanical properties of 3D-printed provisional fixed materials compared to conventional and milled resin materials. The secondary outcome was the mechanical properties variation between the different 3D-printed provisional materials and conditions.

Materials and Methods
Ethical approval was not applicable in the current study because it was exclusively based on the published literature and strictly adhered to research ethics by only using previously published studies ethically approved by original researchers [43,44].

Data Items
The well-known PICO strategy was used to select the focus question of this study. The PICO strategy used was as follows: (1) Population: Dental provisional crown, bridge, or FDP.
(2) Intervention: 3D-printed provisional, interim, or temporary materials. The PICO focus question of the presented review was: "Within the available studies in the literature, do provisional 3D-printed fixed materials have mechanical properties comparable to conventional or milled provisional fixed materials?". The null hypothesis was that 3D-printed provisional prostheses material mechanical properties are not different from the current available milled and conventional provisional fixed materials.

Information Sources and Search Strategy
The used research protocol, which was registered in the research registry (reviewreg-istry1599), implemented a systematic approach to searching the literature for relevant studies, screening selected studies for eligibility, and assessing their quality. The reporting of this systematic review was guided by the standards of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) statement [45,46]. According to Page et al. in 2021 [29], PRISMA 2020 would not assess the quality of systematic reviews; however, familiarity and guidance by its checklist and instructions are crucial to conducting a systematic review that completely captures all recommended information. The reporting of PRISMA in the submitted systematic review follows suit of recently published systematic reviews that also reported guidance by the PRISMA 2020 guidelines without conducting a meta-analysis [47][48][49].
PubMed/Medline was electronically searched using Boolean operators to locate appropriate articles from January 2011 to March 2023 (Table 1). The titles and abstracts identified through the electronic search were exported into a Microsoft Excel sheet. The reference list of the initially included studies was manually searched for any additional relevant studies. The relevant articles were identified, and non-relevant articles were removed.

Eligibility Criteria
The included criteria were in vitro and in vivo studies that investigated mechanical properties with sufficient sample sizes and statistically analyzed results. They should be published in peer-reviewed journals available in English. Studies evaluating the mechanical properties of 3D-printed fixed provisional materials compared to conventional and/or milled provisional fixed materials. The exclusion criteria were review articles, case reports/series, study evaluation properties other than mechanical properties, not mentioning the material name, 3D-printed material used for removable prostheses such as the denture base, denture teeth, or occlusal/night guard prosthesis, non-dental uses of 3D-printed material, and studies without control samples (conventional or milled).

Study Selection and Data Collection Processes
Titles and abstracts were evaluated and selected for appropriateness by two independent investigators (AAA, WMA). Upon identification of a potential abstract, the full text of the article was retrieved. Articles were reviewed and subjected to inclusion and exclusion criteria. Any discrepancies after critical appraisal of the selected articles were resolved by discussion between the two investigators.

Synthesis of the Results
A reading grid was used for data extraction, and the data were summarized in a table form. The systematically extracted data included the author/year of publication, evaluation method, types of materials (Conventional/Milled/printed), and study outcome.

Data Extraction, Quality, and Bias Analysis
Cohen's Kappa was calculated after performing the reviewers' calibration. The cutoff value was set at 85%. By evaluating the risk of bias using criteria modified from earlier studies on basic research publications [50], quality evaluation was carried out. The set of requirements included: (1) Published materials in peer-reviewed journals. (2) Provides statistical analysis. (3) Randomization of test groups or controls. (4) Blinded evaluation. (5) Estimating the sample size before the experiment. (6) Examining the dose-response relationship. (7) A declaration of conformity to regulatory standards. (8) Objective congruence between the research and the relevant study. Depending on whether or not the parameters were reported, the quality level varied from low to high, reflecting our confidence that the estimation of the effect was correct. A positive choice with a score of 6 to 8 was deemed to have a low risk of bias, while one with a score of 3 to 5 was deemed to have a moderate risk of bias. A choice that was 1-2 positives was deemed to have a high bias risk.

Study Selection
A total of nineteen studies were included in this systematic review. Fifteen studies were retained out of eighty-five studies, and four articles were manually added from the reference list of the retained articles. A flow chart of the selection process guided by the PRISMA 2020 statement was formulated. Figure 1 illustrates the process of identification of the relevant articles in a flowchart.

Synthesis of the Results
A reading grid was used for data extraction, and the data were summarized in form. The systematically extracted data included the author/year of publication, e tion method, types of materials (Conventional/Milled/printed), and study outcome

Data Extraction, Quality, and Bias Analysis
Cohen's Kappa was calculated after performing the reviewers' calibration. The value was set at 85%. By evaluating the risk of bias using criteria modified from studies on basic research publications [50], quality evaluation was carried out. The requirements included: (1) Published materials in peer-reviewed journals. (2) Pr statistical analysis. (3) Randomization of test groups or controls. (4) Blinded eval (5) Estimating the sample size before the experiment. (6) Examining the dose-re relationship. (7) A declaration of conformity to regulatory standards. (8) Objectiv gruence between the research and the relevant study. Depending on whether or n parameters were reported, the quality level varied from low to high, reflecting our dence that the estimation of the effect was correct. A positive choice with a score o was deemed to have a low risk of bias, while one with a score of 3 to 5 was deem have a moderate risk of bias. A choice that was 1-2 positives was deemed to have bias risk.

Study Selection
A total of nineteen studies were included in this systematic review. Fifteen s were retained out of eighty-five studies, and four articles were manually added fr reference list of the retained articles. A flow chart of the selection process guided PRISMA 2020 statement was formulated. Figure 1 illustrates the process of identifi of the relevant articles in a flowchart.   Table 2 represents the outcome of the risk of bias assessment used in this systematic review.  Most of the studies were published in the last 3 years. Out of the nineteen included studies, fourteen articles showed a low level of bias, and five articles showed a moderate level of bias. None of the included articles reported blindness of the investigator during the fabrication of the specimens or testing. Moreover, specimen randomization was only applied in nine studies. In addition, sample power calculation before testing was only mentioned in five articles.

Study Characteristics
During data extraction, heterogeneity was observed in the methods adopted, the technique of fabrication of the samples, the materials used, and the parameters studied. These studies investigated different aspects of the mechanical properties of 3D-printed provisional resin materials, with a tendency to focus on the evaluation of fractural strength (FS), wear resistance (WR), surface roughness (SR), and hardness (VH and KH) properties.

Results of Individual Studies
The results of individual studies were summarized in Table 3. Table 3. Summary of included studies with types of testing methods, types of specimens, materials used, and main outcomes.

Ref
Testing

Discussion
Provisional restorations are an important step in fixed treatments, especially when inserting the final prosthesis during the same visit might be difficult. One of the recently adopted techniques for fabricating provisional crowns and FDP is 3D-printed provisional materials. This systematic review focused on evaluating the mechanical properties of the available 3D-printed resin materials and compared them to milled and conventional provisional resin materials. The null hypothesis that there is no significant difference in flexural strength, wear resistance, and surface roughness among the available 3D-printed fixed provisional materials was accepted. Furthermore, 3D-printed resin material showed generally good and comparable results to milled and conventional methods of fabrication of provisional materials.
In a recent systematic review in 2022, [41] mechanical properties excluding microhardness, toughness, and resilience, of 3D-printed provisionals were found to be superior to milled and conventional methods of fabrication of provisional crowns and FDPs. However, most of the pooled estimates included in that review were inconclusive, with very high heterogeneity, which restricted the applicability of the performed meta-analysis. Further investigation of factors affecting the mechanical properties was suggested [41]. In the current review, several factors were identified that influence the mechanical properties of 3D-printed materials: thickness of the printed layer, post-curing techniques, shrinkage between the layers, curing speed, intensity, angle, post-polymerization time and temperature, and printing direction [42]. Alharbi et al. [35] found that the compressive strength of 3D-printed materials improved by printing the layers perpendicular to the load direction. After printing two different provisional materials at 0, 45, and 90 degrees. Derban et al. [31] determined a significant impact of printing orientation on flexural strength.
A comparison of 3D printing resin mechanical properties with conventional and milled FDPs using three-unit FDP was performed by multiple studies [56,60,62]. There was an agreement of a higher flexural strength in milled provisional FDPs compared to conventionally fabricated provisional FDPs. The milled materials have higher fillers packed under higher temperatures and pressure, which result in lower porosity, voids, and residual monomers compared to 3D printing and conventional resins [62]. However, a printing orientation of 0 • and 30 • significantly improves the flexural strength of three-unit FDPs [60]. Despite the high variability between the two studies assessing the flexural strength of FDPs in terms of the post-curing process and cementation, milled FDP had a higher flexural strength compared to DLP FDP when using the same cleaning agent of isopropanol [62,65]. Investigating the effect of cementation is a crucial aspect, because cementation increases the FDP flexural resistance by evenly distributing the external forces throughout the specimen [3], and it is more clinically relevant than measuring the flexural strength without cementation.
All of the studies that evaluated flexural strength included in this systematic review had results higher than 50 MPa, which is the minimum flexural strength of the recommended fixed provisional prosthesis according to ANSI/ADA specification no. 27. The process of enhancing the flexural strength of printed polymers is still in progress. Aati et al. [29] modified the printable resin by adding zirconia oxide (ZrO 2 ) nanoparticles at different concentrations. They found superior mechanical properties compared to the unmodified printable resin.
Multiple studies that compared the wear resistance of 3D-printed materials showed less wear volume loss and displayed smoother surfaces compared to both milled and conventional provisional materials [62,63,67,68,70]. During printing, printers have the ability to deposit layers up to a tenth of a micromillimeter, which results in a product with a smoother surface and minimizes the polishing time in comparison to the milling [27]. There are many factors to be taken into consideration when evaluating wear tests. For instance, the machine used to perform the test, the media, the size and shape of the indenter, the load applied, and the number of cycles the samples undergo, are all factors that can create variabilities between the studies that affect our ability to quantitatively assess the data and draw a strong conclusion.
Hardness was evaluated by four studies with contrasting results [62,63,67,68,70]. Hardness was assumed to be higher in milled provisional resin materials due to higher cross-linked monomers and the presence of filler loading. Thus, increasing the hardness compared to conventional and 3D-printed materials. However, the SLA technique of printing had a comparable result to the milled technique [55]. This could be due to the printing technique mechanism itself, which allows printing successive layers under the controlled penetration of a UV laser beam [56].
There are still many concerns regarding 3D-printed materials that might influence their mechanical properties: cleaning the object of the remnants of uncured resin after the printing [62,71], and the ability to reline the shells and repair fractures [72,73]. The impact of post-curing techniques, the time taken, and the mechanical and physical accuracy of the 3D-printed materials [74][75][76]. Reymus et al. [76] found a degree of conversion influenced by the strategies of the curing used post printing, which will influence the mechanical properties of the material. Additionally, the maximum number of units for the fabrication of FDP and cleaning solutions must be thoroughly investigated before any recommendations can be made.
This systematic review solely focused on the mechanical properties of 3D-printed materials and did not consider any other biological, chemical, or physical properties. Furthermore, the search methodology was restricted to published studies in the English language. Additionally, review articles in the Cochrane database, IEEE, and/or preprints were not included in the search, which could have limited the results. Another limitation is the fact that conducting and reporting a meta-analysis can be challenging due to significant differences between the included studies, making it difficult to combine and quantitatively analyze them. This heterogeneity can include indirect comparisons and studies that are too dissimilar to be combined and effectively analyzed [77] using different testing apparatuses, testing conditions, and/or specimen fabrication methods (crowns, FDP, implant crowns, bar-shaped and disc-shaped). Therefore, the authors suggest conducting additional studies that cover various aspects of the material properties and different clinical scenarios.

Conclusions
As more dental practitioners and technicians become interested in 3D-printed provisional materials, it is crucial to ensure that these materials have the best possible mechanical properties before they replace older methods such as milled and conventional provisional materials. Based on our systematic review, we have found that within its limitations, using 3D-printed provisional materials shows great promise for creating provisional crowns and FDP.